With miniaturization and high-density mounting of electronic components in recent years, BGA (a ball grid array) and CSP (a chip size package) technologies have come into use when mounting electronic components on a printed-wiring board or the like. Electrodes employed in those technologies also are getting increasingly miniaturized.
In joining these electronic components, solder bumps are initially formed in a plurality of electrodes arranged on semiconductor substrates, electronic components, printed-wiring boards or the like. Formation of the solder bumps on the electrodes on the electronic members is performed by allowing solder balls to adhere to respective electrodes using adhesive force of flux and then heating the electronic members to a high temperature to reflow the solder balls. Semiconductor substrates etc. and printed-wiring boards etc. are joined together through the solder bumps. Here, the term “solder bump” means a solder formed in a hemispherically-raised shape on a plated layer on a copper or aluminum wiring electrode.
In order to minimize an influence upon the environment at the time of disposal of discarded electronic devices, solder alloys used for electronic devices have also come to require lead-free ones. Examples of such lead-free solder alloys include, as a binary alloy, the one comprised of Sn containing 3.5% Ag, which has been extensively used, since that composition provides eutectic composition, indicating a comparatively low melting point as low as 221 deg C.
With the high-density mounting of electronic components in recent years, surface mounting and BGA mounting have progressed particularly in the fields of notebook computers, video cameras, mobile phones, automobile-mounted electronic members and electronic equipment and the like so that scaling down in a pad area of a substrate electrode has been progressing rapidly, thus having forced the amounts of solders used in solder joints to be decreased. That is, junction areas in soldered portions have been decreased, thus resulting in increased stresses being applied to the junction. Further, since the high-density mounting has caused communication devices to become increasingly sophisticated and miniaturized, portability of communication devices also has progressed rapidly. Additionally, economic activities having been expanded on a global scale has caused such devices to be used in places no one has ever though of, such as burning deserts or under extreme cold conditions in polar zones and highlands, which requires that soldering mounting be designed, taking the fact that the soldered junctions may be exposed to an even severer environment into consideration. Accordingly, demands for improvement in fatigue resistance of solder materials have increased further. In patent document 1, there is disclosed, as a lead-free solder for electronic equipment, a high-temperature solder consisting of 3.0 to 5.0% of Ag, 0.5 to 3.0% of Cu, and a balance of Sn, which is excellent in thermal fatigue resistance. With respect to a content of Ag, the document teaches that the addition of Ag exerts a significant effect on improvement in the thermal fatigue resistance, whereas if an additive amount of Ag is 3.0% or below, the improvement effect is insufficient.
Besides, in the case of portable digital products such as a mobile phone or the like, it is necessary to assume such situation that the products may be accidentally dropped on a floor surface or struck against the same during the use thereof, in view of the specific manner they are actually used. Accordingly, they are required to have impact resistance sufficient to cause no destruction to the soldered junctions of electronic components used, even if they are subjected to impacts like the above-mentioned. Whereas, according to the conventional fatigue-resistant solder alloys, improvement of the fatigue resistance has been realized mainly by increasing the strength of the solder, so that there has been a tendency that the impact resistance rather deteriorates. In order to improve the impact resistance and vibration resistance of soldered junctions, an alloy excellent in ductility is most effectively used as a soldering alloy of the joints.
Further, automobile-mounted electronic components are required to have sufficient durability against vibrations generated during the travel. Accordingly, they are required to have vibration resistance enough to keep the soldered junctions of the electronic components from being damaged even against such repetitive vibrations. Whereas, according to the conventional fatigue-resistant solder alloys, improvement of fatigue resistance has been realized mainly by increasing the strength of the solder, so that there has been a tendency that the vibration resistance rather deteriorate. In order to improve the vibration resistance of soldered junctions, an alloy excellent in ductility is most effectively used as a soldering alloy of the joints, as is the case with the above-mentioned improvement of the vibration resistance.
In patent document 2, there is disclosed a lead-free solder alloy which has a lower Ag content than that disclosed in the patent document 1 and is excellent in drop-impact resistance, wherein the lead-free solder consists of 1.0 to 2.0% by mass of Ag, 0.3 to 1.5% by mass of Cu and a balance of Sn and unavoidable impurities. As a result, the lead-free solder according to the patent document 2 enables the lead-free solder to be provided at lower cost than by the conventional ones, thus realizing extremely excellent thermal fatigue resistance and impact resistance at the same time. The patent document 2 teaches that for the purpose of improving strength of the solder alloy, 0.05 to 1.5% by mass of Ni or 0.005 to 0.5% by mass of Fe may preferably be added.
In patent document 3, there is disclosed a lead-free solder alloy for improving impact resistance and heat cycle resistance, wherein the lead-free solder consists of 0.01 to 1% by mass of Sb, 0.01 to 0.5% by mass of Ni, and a balance of Sn, with additives of 0.01 to 5% by mass of Ag and/or 0.01 to 2% by mass of Cu. The patent document 3 teaches that Sb has the effect of improving impact resistance, while Ni has the effect of improving heat cycle resistance, and that the addition of Cu further improves the impact resistance, while the addition of Ag further improves the heat cycle resistance.
In patent document 4, there is disclosed a solder alloy which is thermally stable, excellent in joining property, and excellent in strength, wherein the solder alloy primarily consists of Sn, further containing: 1.0 to 4.0% by weight of Ag; 2.0% or less by weight of Cu; and 1.0% or less by weight of Ni. The patent document 4 teaches that Cu improves the strength and heat resistance of an alloy without impairing wettability, and that the addition of Ni increases the thermal stability of an alloy, improving the strength and thermal fatigue resistance thereof, while suppressing the formation of intermetallic compound which causes joining strength to be reduced when joined to a cupper substrate.
In patent document 5 is disclosed an invention in which Sn-4.7% Ag-1.7% Cu solder alloy comprises an additive element selected from among Ni, Fe and Co. The patent document 5 teaches that each additive element may be present in an amount of at least about 0.01 weight % so that the Cu substrate/solder interface morphology is improved, and particularly the thickness of the as-solidified intermetallic interface is reduced.    Patent Document 1: Japanese unexamined patent application publication No. H5-50286    Patent Document 2: Japanese unexamined patent application publication No. 2002-239780    Patent Document 3: Japanese unexamined patent application publication No. 2004-141910    Patent Document 4: Japanese registered patent publication No. 3296289    Patent Document 5: Japanese unexamined patent application publication No. 2001-504760